Appropriateness of environmental impact distribution methods to model blast furnace slag recycling in cement making

• We compare 3 types of Portland cement with different contents of blast furnace slag.
• We go beyond previous studies on allocation by applying the system expansion approach.
• System expansion allows for consideration of improvements at whole-system level.

PurposeEnvironmental impact allocation has been noted as one of the life cycle assessment's (LCA) most controversial methodological issues given that it highly influences the study's final result. This paper analyses the appropriateness of available multifunctional modeling methods to distribute environmental loads between pig iron and bfs produced in the steelmaking process, and the influence that modeling choices have on LCA results for different blended cement types commercialized in Brazil.MethodsAllocation by mass and by economic value, as well as system expansion, are examined for ordinary Portland cement and two types of blended cements with higher ggbfs contents as clinker replacement. The support platform used for performing the LCA was SimaPro 7.3, and the impact evaluation method was CML 2001 (baseline). The data for the production processes’ modeling came from national and/or local reports; when national data were unavailable, the corresponding processes found in the SimaPro built-in Ecoinvent database were adapted to better represent the Brazilian context.Results and discussionAs expected, impact allocation based on mass induces large impacts on bfs and the environmental loads of blended cements gradually increase with the increased bfs content. A similar trend was observed for economic allocation, except for global warming and terrestrial ecotoxicity, which are particularly sensitive to the allocation procedure choice. In the system expansion approach, impact values in all categories decreased with increased ggbfs content in cement.ConclusionImpact distribution across products is a legitimate way for an industry to be held accountable only for the true impacts of a given process. Each method presents its advantages and disadvantages. Independently of the criterion chosen, the conceptual limitations of allocation methods are that they do not look beyond impacts partitioning, and awkward ratios between physical characteristics and market value blur the vision and distort results. In our case, the system expansion approach precisely modeled the studied processes, following a complete and conceptually consistent description, which also allows for consideration of potential improvements at whole-system level.